I'm not sure what units the question intended so I put both. For the first one, the equation for density is mass/volume so I divided the 20g by 5cm^3 to get 4g/cm^3. Just in case those aren't the intended units for volume and is instead liters, one cm^3 equals one ml. In addition, there are 1000ml in every liter so you can determine the amount per liter by multiplying 4g/ml by 1000 to get 4000g/L or 4kg/L.
